In digital mobile communication systems using a CDMA (Code Division Multiple Access) method, users are separated on the basis of spreading codes. At a base transceiver station, symbols to be transmitted to a user are multiplied by a spreading code, which determines a spreading factor. Spreading codes are selected e.g. from a code tree determining the codes, wherein the spreading codes are mutually orthogonal in order to minimize cross-correlation between codes. A code tree can be constructed e.g. such that the highest level, where the spreading code is one, is only provided with one code, which branches out into two mutually orthogonal codes on a lower, i.e. second, level. The two second-level codes thus generated are identical as far as the first half of the code is concerned, but the second halves of the codes differ from each other in being mutually orthogonal. Each of the second-level codes is further branched into two new codes on a lower level, all codes on a same level being mutually orthogonal. The codes are also orthogonal with codes on different levels of the code tree as seen from the point of view of spreading, i.e. codes can be allocated from the same tree to users using different transmission rates. Spreading codes are often allocated from the same spreading code tree to users in a particular system, and interference between different base transceiver stations is prevented by means of scrambling codes determining the base transceiver stations. All spreading codes provided by the code tree are thus available to the base transceiver station or even to a particular transmission sector. User signals generated by means of the spreading codes are combined into a broadband combination signal, which enables the available restricted radio resources to be utilized efficiently.
The combination signal generated by combining user signals includes occasional power peaks that interfere with the operation of the system. Furthermore, the implementation of an amplifier which produces a power peak is not very cost-effective in practice, so the combination signal is often subjected to clipping wherein power peaks exceeding a given threshold value are clipped off from the signal. A drawback to clipping is that the signal becomes more difficult to receive because the separating capability provided by the orthogonality of the users' spreading codes decreases.
In mobile communication networks, such as in the known UMTS (Universal Mobile Telephony System) system, the spreading codes can be selected for users from several different levels when the spreading factor ranges between 4 and 512 such that spreading codes for users in need of lower transmission rates are allocated from higher spreading factor levels. In prior art solutions, on a certain spreading factor level, such as on level 256, spreading codes are selected in an order starting e.g. from the first code in order.
Since the spreading codes are selected from the code tree in such an order that a new code to be selected is often a code adjacent to the code previously selected, the codes selected for use often include codes whose first parts are congruent with those of the codes already being used. When the spreading codes in use have a large portion in common, it becomes more difficult to identify user information in a combination signal. Such a set of codes wherein the codes are selected in a particular order is particularly weak when clipping is performed on a part of a signal wherein the codes differ from each other and congruent parts remain. It is thus obvious that a need exists to provide a method and an apparatus for selecting spreading codes such that the capacity for separating user signals remains as high as possible, particularly when clipping a combination signal so as to eliminate power peaks.